1. Field of the Invention
The present invention relates to a hysteresis characteristic setting device and a method, and more particularly to a device and a method for setting a hysteresis characteristic with respect to an input signal in order to prevent, for example, noises from being detected as an input signal.
2. Description of the Related Art
Conventionally, as a circuit for setting a hysteresis characteristic with respect to an input signal in order to prevent noises from being detected as an input signal, known are a logic circuit and a comparator circuit which have a hysteresis characteristic as typified by a Schmitt trigger circuit.
FIG. 5 exemplarily shows the configuration of a Schmitt trigger circuit consisting of an operational amplifier and plural resistors. The Schmitt trigger circuit outputs an H-level signal when the voltage of an input signal is lower than a preset upper limit (of a hysteresis voltage), and outputs an L-level signal when the input signal voltage is higher than the upper limit. Thereafter, the circuit continues to output the L-level signal when the input signal voltage is higher than a preset lower limit (of the hysteresis voltage), and outputs the H-level signal when the input signal voltage is lower than the lower limit. In this way, in the Schmitt trigger circuit, when the input signal voltage is once higher than the upper limit, the level of the output signal is not changed as long as the voltage becomes lower than the lower limit. Similarly, when the input signal voltage is once lower than the lower limit, the level of the output signal is not changed as long as the voltage becomes higher than the upper limit. In the specification, the operation is referred to as an operation of setting a hysteresis characteristic with respect to an input signal, and a device for performing the operation is referred to as a hysteresis characteristic setting device. The operation of setting a hysteresis characteristic prevents an effect due to a voltage change of the input signal caused by noises (specifically, a voltage change to the upper limit when the input signal voltage is raised, or that to the lower limit when the voltage is lowered) from being produced. Therefore, noises are eliminated from the input signal. The waveform of the input signal may be shaped into a rectangular one by the operation.
Patent literature 1: JP-UM-B-5-28521
As described above, a logic circuit or a comparator circuit which sets a hysteresis characteristic as typified by a Schmitt trigger circuit is configured by an operational amplifier or transistors and resistors. Therefore, such a circuit has a large number of components, with the result that the production cost is increased and a large installation space is required.
It is an object of the invention to provide a hysteresis characteristic setting device configured by components which are smaller in number than those of a logic circuit or a comparator circuit for setting a hysteresis characteristic.